On Thu, Aug 12, 2021 at 9:41 AM Hank Nussbacher <hank@interall.co.il> wrote:
> On 12/08/2021 17:59, William Herrin wrote:
> > If you prune the routes from the Routing Information Base instead, for
> > any widely accepted size (i.e. /24 or shorter netmask) you break the
> > Internet.
>
> How does this break the Internet? I would think it would just result in
> sub-optimal routing (provided there is a covering larger prefix) but
> everything should continue to work. Clue me in, please.
A originates 10.0.0.0/16 to paid transit C
B originates 10.0.1.0/24 also to paid transit C
C offers both routes to D. D discards 10.0.1.0/24 from the RIB based
on same-next-hop
You peer with A and D. You receive only 10.0.0.0/16 since A doesn't
originate 10.0.1.0/24 and D has discarded it.
You send packets for 10.0.1.0/24 to A (the shortest path for
10.0.0.0/16), stealing A's paid transit to C to get to B.
Unless A filters C-bound packets purportedly from 10.0.1.0/24. B
doesn't currently transit for A so from B's perspective that's not an
allowed path. In which case, your path to 10.0.1.0/24 is black holed.
Bill, I beg to respectfully differ, knowing that I'm just a researcher and working `for real' like you guys, so pls take no offence.
I don't think A would be right to filter these packets to
10.0.1.0/24; A has announced
10.0.0.0/16 so should route to that (entire) prefix, or A is misleading its peers.
If A doesn't, though, then B receives a packet from A to
10.0.1.0/24. Unless B is filtering based on the specific IP prefixes of A - which seems to me unlikely - then B has no way to know that this packet is from `you' rather than from A itself (or a customer of A). So B will carry this traffic, imho.
So A is just paying for the traffic since it announced the prefix.
Such situations, to best of my knowledge, actually happen on the Internet when a subprefix is filtered for different reasons. We observed it happens with ROV , in our ROV++ simulations, but I'll refrain
from attaching the URL again so not to be `plugging' that paper (and since I'm lazy to look it up hhh)
have great day and I'll be happy to learn if I'm wrong. Amir